1. Field of the Invention
The present invention relates to a diamond film field-effect transistor (hereinafter referred to as FET) wherein a semiconducting layer functioning as a channel layer is formed of a highly-oriented diamond film.
2. Prior Art
Diamond has a large band gap (5.5 eV), high thermal conductivity (20 W.K/cm), high saturation carrier mobility (2000 cm.sup.2 /V.sec for electrons and 2100 cm.sup.2 /V.sec for holes) and a high dielectric breakdown voltage (10.sup.7 V/cm).
Since it is possible to make a semiconducting diamond by adding appropriate impurities to diamond, diamond has drawn attention for application to various fields, such as electronic devices operating under high temperature and radiation, and high power and high frequencies.
There are many proposals for the structure of field-effect transistors (FETs) using diamond films (Japanese under Provisional Publication sho 64-68966, and hei 1-158774, 3-94429, 3-12966, 3-160731, and 3-263872).
Diamond film FETs disclosed by these prior art references are all fabricated by either single crystal diamond substrates, homoepitaxial diamond films grown on single crystal diamond substrates or polycrystalline diamond films as constituents.
Single crystal diamond has a disadvantage for practical use because the integration of device elements is impossible due to its small surface area (several mm.sup.2). Further, single crystal diamonds are expensive and therefore the manufacturing cost becomes very high. In addition, there exist crystal defects in natural and synthesized single crystal diamonds and therefore the electrical characteristics have not yet reached the level of theoretical characteristics for single crystal diamond.
Recent advancement of diamond film deposition technology has made it possible to grow a uniform polycrystalline diamond film on a non-diamond substrate over 4 inch wafer and at low cost. Therefore, production of FETS using poly-crystalline diamond films can lead to high integration of diamond film FETs. However, currently produced polycrystalline diamond films contain many grain boundaries and the surface of the film is very rough.
The grain boundaries cause carrier scatterings which results in a significant reduction in the hole mobility, and can cause current leakage. If the polycrystalline diamond film is under high temperature in air, is oxidized and graphitized along the grain boundaries. The crystal defects also cause carrier scatterings which also results in a decrease of carrier mobility. The roughness of the film surface causes a heterogeneity in the electric field in electronic devices.
Therefore, although, polycrystalline diamond films have an advantage for device production due to its large area, the characteristic of the fabricated devices are far behind the commercially acceptable level.